Yes I will have the system checked for leaks and the evacuation and the charging completed. But I have also seen that it is difficult to put r12 into the vehicle since nobody carries the equipment now.

If you can fix the system and eliminate all leaks keeping R12 is your best path. If you switch to 134 you will need to replace all lines to barrier lines in addition to valve change. It might be hard to find people willing to work on an R12 system in Cali, which would tend to drive you to switch.

Changing the barrier lines is "optional"......yes, R134 may "leak" due to it's different molecular structure in original barrier lines, but it is something you can wait and see and then address as necessary. Probably always cheaper to top up with R134 anyway.
My system is working well after a conversion a year ago and I did not change the barrier lines.

If you can fix the system and eliminate all leaks keeping R12 is your best path. If you switch to 134 you will need to replace all lines to barrier lines in addition to valve change. It might be hard to find people willing to work on an R12 system in Cali, which would tend to drive you to switch.

Initially there was a "requirement" that when converting to R-134a you should switch to barrier hoses (R-134a, smaller molecules, blah, blah, blah, etc ), but later laboratory testing indicated that non-barrier hoses "pre-conditioned" through the use of R-12 and R-12 lubricant did not leak at a significant level.

According to the EPA and DuPont statements available on the internet.

In addition, some manufacturers still use non-barrier hoses on the suction, low pressure side of the system, a clear indication that system operating (excessive ?) pressure plays a significant part in the question of leakage or no.

Which is why the EPA REQUIRES that a pressure switch be added to legacy systems such as ours during an upgrade to R-134a. Many older systems, of our era, up to the early 90's, incorporated an ~500 PSI HPRV, High Pressure Relief Valve, to vent excessive high side system refrigerant pressures to atmosphere.

Vent R-12 to atmosphere..?? Yes, that is correct, R-12 was then relatively inexpensive and was no yet known to be adverse to our atmosphere.

But our systems have no HPRV...

Did Porsche consider non-barrier hose permeation under excessive pressure to be the solution?

Our system's, under certain specific circumstances, are theoretically subject to excessive high side system pressures....

With the evaporator iced over, for instance, not exactly an uncommon situation. Our thermostatic control switches, at maximum cooling, full CW, open the compressor circuit only once the sensor detects a temperature slightly below freezing. Some switches might be so far out of calibration that the compressor clutch "off" action is well below freezing.

ICE is a GOOD insulator, once the evaporator is mostly frozen over the sensor will/might be insulated against temperatures below 32dF.

Then the compressor runs continuously until you discover the freeze-up condition, lack of system airflow, and manually switch the system off long enough to defrost, DE-ICE, the evaporator.

What high side pressures might you then see at 3000 RPM.

Most qualified A/C shops will convert your system to R-134a for about $200. That does not include an R/D upgrade nor a pressure switch installation/wiring. $300 will get you both of those, <$30 for the R/D, and <$20 for a pressure switch with R-134a adapter.

Changing the barrier lines is "optional"......yes, R134 may "leak" due to it's different molecular structure in original barrier lines, but it is something you can wait and see and then address as necessary. Probably always cheaper to top up with R134 anyway.
My system is working well after a conversion a year ago and I did not change the barrier lines.

But it does have a Red Dot 71R7500 pressure switch, 325 PSI compressor clutch cutout. Lots of R-134a pressure switches in the market, Griffiths, etc, have a high pressure cutout of >400 PSI. Probably fine with barrier hoses, but seems a bit high for our non-barrier hoses given the Red Dot option is available.

I've never heard of having to replace/update the pressure switch on a 964 due to a refrigerant switch.

The 964 pressure switch resides in tight confines on the top of the HVAC suitcase, between servos and stuff. I'm not sure any switch is just gonna fit in there unless it is a thin design like the stock piece.